1. Field of the Invention
The present invention relates to image recording apparatuses, and more particularly to a recording head having a recording-head substrate.
2. Description of the Related Art
In recording heads mounted in conventional ink-jet recording apparatuses, electrothermal transducers (heaters) and driving circuits are provided integrally on a substrate by a semiconductor process technology, such as that disclosed in U.S. Pat. No. 6,290,334.
Furthermore, various techniques have been proposed which detect the state of the substrate. For example, techniques to detect the temperature of a substrate based on an output from a temperature-detecting element provided on the same substrate.
FIG. 11 is a block diagram showing a configuration of a known ink-jet recording-head substrate (hereinafter simply referred to as a “substrate”) having a temperature-detecting element.
Referring to FIG. 11, heaters serving as electrothermal transducers for heating and discharging ink, and driving circuits therefor are provided integrally with an ink-jet recording-head substrate 100 by a semiconductor process technology. The ink-jet recording-head substrate 100 includes driver and heater arrays 101 in which a plurality of heaters and driver circuits are arranged, an ink supply channel 102 for supplying ink from the back side of the substrate 100, shift registers (S/R) 103 for temporarily holding external input recording data, input circuits 104 including buffer circuits for inputting digital signals from the recording apparatus to the shift registers 103, and decoder circuits 107 (which will be described later). The ink-jet recording-head substrate 100 also includes signal lines 105 for sending a signal for selecting any segment of the driver and heater arrays 101 from the shift registers 103 and the decoder circuits 107, the decoder circuits (decoders) 107 for selectively driving a desired heater block of the driver and heater arrays 101, and a temperature-detecting element 120 for detecting the temperature of the substrate 100.
FIG. 12 is an equivalent circuit diagram of a circuit used to drive one heater and one driver (one segment) in order to supply a current to the heater for ink discharging.
The circuit shown in FIG. 12 includes an AND circuit 901 for obtaining an AND between a block select signal (block select) sent from the decoder circuit 107 to select heaters from a plurality of blocks, and recording data (bit select) transferred to the shift register 103 and then held according to a latch signal in order to selectively drive the heaters, an inverter circuit 902 for buffering an output from the AND circuit 901, a power line (VDD) 903 serving as a power supply for the inverter circuit 902, a power line (VHT) 904 serving as a power supply connected to an inverter circuit 908 (which will be described later) in order to supply a gate voltage for a driver transistor 907 (which will be described later), a power line (VH) 905 serving as a power supply for heater driving, a heater 906, the driver transistor 907 for applying a current to the heater 906, and the inverter circuit 908 serving as a buffer that receives an output from the inverter circuit 902.
FIG. 13 is an equivalent circuit diagram of a circuit corresponding to one bit of a shift register (S/R) that temporarily stores recording data and a latch circuit.
FIG. 14 is a timing chart showing a series of operations of transferring recording data to a shift register (S/R) and of applying a current to a heater.
In synchronization with a clock pulse (CLK) input to a terminal 1001 shown in FIG. 13, recording data (DATA) is supplied to a terminal 1003. A shift register temporarily stores the recording data, and a latch circuit holds the recording data according to a latch signal (BG) applied to a terminal 1005. When the clock pulse and the latch signal are input, inversion signals (ICLK, IBG) corresponding thereto are also input from terminals 1002 and 1004, respectively, in order to ensure high reliability during a high-speed operation.
Subsequently, an AND between a block select signal (block select) for selecting heaters divided in a plurality of blocks and the recording data (DATA) held by the latch signal (BG) is obtained, and a heater current is applied in synchronization with a heat enable signal (HE) that is input from a terminal 1007 to directly determine the current-driving time. These operations are repeated for each block to perform recording.
In a general ink-jet recording-head substrate using electrothermal transducers (heaters), when recording is repeated with heat generated by driving the heaters, the temperature of the substrate rises. In order to prevent the temperature rise from affecting the ink discharging characteristics and from worsening the recording condition, the temperature of the substrate is monitored at regular intervals, and the driving method is appropriately controlled in accordance with the temperature. In this case, the most typical method for monitoring the temperature of the substrate is to read the temperature characteristic of a voltage generated by passing a fixed current through a diode provided on the same substrate.
In general, the change of the current-voltage characteristic of the diode depending on the temperature is given by the following expression:VF=(k·T/q)In(IF/IS)Since the characteristic is almost directly and exclusively determined by a production process of the substrate and can be estimated beforehand, the temperature of the substrate can be detected by monitoring the voltage output from the diode.
FIG. 15 is an equivalent circuit diagram of a diode conventionally used as a temperature-detecting element.
In FIG. 15, IN denotes an input-voltage terminal, OUT denotes an output-voltage terminal, and 1200 denotes a diode.
FIG. 16 is a graph showing voltage-temperature characteristics of a typical diode.
As shown in FIG. 16, the resistance of the diode decreases as the temperature rises. Therefore, when a constant current flows through the diode, the output voltage (OUT) decreases as the temperature rises, and increases as the temperature drops.
A diode serving as a temperature-detecting element is connected to a temperature output terminal (Temp) on the substrate shown in FIG. 11. The temperature of the substrate is detected by reading, from the temperature output terminal, the voltage generated by the application of a constant current to the diode. In this case, at least one temperature output terminal needs to be provided on the substrate so as to serve as an interface for external connection.
When the above-described conventional method for detecting the temperature is applied to an ink-jet recording head having a plurality of substrates, it is necessary to monitor at least one temperature output terminal provided in each of the substrates in order to detect the temperature condition of the substrate. This means that it is necessary to provide a number of signal output terminals for external connection corresponding to the number of the substrates in the recording head.
However, such an increase in number of terminals increases the number of electrical contacts in the recording head, and the area of the recording head and the number of lines are increased. Moreover, a processing circuit for separately processing the temperatures of the substrates is complicated. As a result, the costs of the recording head and a recording apparatus using the recording head are increased.